Strong Shubnikov-de Haas Research Articles

Magnetotransport properties of ternary tetradymite films with high mobility

(Bi,Sb)2(Te,Se)3 tetradymite materials are among the most efficient for thermoelectric energy conversion, and most robust for topological insulator spintronic technologies, but should possess rather disparate doping properties to be useful for either technology. In this work, we report results on the molecular beam epitaxy growth of p-type (Bi0.43Sb0.57)2Te3 and n-type Bi2(Te0.95Se0.05)3 that can contribute to both technology bases, but are especially useful for topological insulators where low bulk doping is critical for devices to leverage the Dirac-like topological surface states. Comprehensive temperature, field and angular dependent magnetotransport measurements have attested to the superior quality of these ternary tetradymite films, displaying low carrier density on the order of 1018 cm−3 and a record high mobility exceeding 104 cm2 V−1 s−1 at 2 K. The remarkable manifestation of strong Shubnikov–de Haas (SdH) quantum oscillation under 9 T at liquid helium temperatures, as well as the analyses therein, has allowed direct experimental investigation of the tetradymite electronic structure with optimized ternary alloying ratio. Our effort substantiates tetradymites as a critical platform for miniaturized thermoelectric cooling and power generation in wearable consumer electronics, as well as for futuristic topological spintronics with unprecedented magnetoelectric functionalities.

Transport properties of a 1000 nm HgTe film: the interplay of surface and bulk carriers

We report on systematic study of transport properties of a 1000 nm HgTe film. Unlike thinner and strained HgTe films, which are known as high-quality three-dimensional topological insulators, the film under study is much thicker than the limit of pseudomorphic growth of HgTe on a CdTe substrate. Therefore, the 1000 nm HgTe film is expected to be fully relaxed and has the band structure of bulk HgTe, i.e. a zero gap semiconductor. Additionally, the system is characterized by the bands inversion, so that the two-dimensional topological surface states (TSSs) are expected to exist. To check this claim we studied classical and quantum transport response of the system. We demonstrate that by tuning the top-gate voltage one can change the electron-dominating transport to the hole one. The highest electron mobility is found to be more than cm2 Vs−1. The system exhibits Shubnikov-de Haas (SdH) oscillations with a complicated pattern and shows up to five independent frequencies in corresponding Fourier spectra. These Fourier peaks are attributed to the TSSs, Volkov-Pankratov states and spin-degenerate bulk states in the accumulation layer near the gate. The observed peculiarities of the quantum transport are the strong SdH oscillations of the Hall resistance, and the suppressed oscillatory response of the TSSs.

Pressure tuning of the Berry phase in BaMnSb2

The Sb square net in ${\mathrm{BaMnSb}}_{2}$ is reported to host nearly massless Dirac fermions. At ambient pressure, ${\mathrm{BaMnSb}}_{2}$ shows strong Shubnikov--de Haas (SdH) oscillations with a $\ensuremath{\pi}$ Berry phase. In this work, we study ${\mathrm{BaMnSb}}_{2}$ under different pressures. It is found that the frequency of the SdH oscillation remains almost unchanged, hence the cross-section area of the Fermi surface does not change much with pressure. Notably, there is a sudden change in the phase factor of the oscillations around a critical pressure of $P=$ 2.5 GPa, which suggests a possible topological phase transition in ${\mathrm{BaMnSb}}_{2}$.

Tensile strained gray tin: Dirac semimetal for observing negative magnetoresistance with Shubnikov–de Haas oscillations

The extremely stringent requirement on material quality has hindered the investigation and potential applications of exotic chiral magnetic effect in Dirac semimetals. Here, we propose that gray tin is a perfect candidate for observing the chiral anomaly effect and Shubnikov-de-Haas (SdH) oscillation at relatively low magnetic field. Based on effective $k.p$ analysis and first-principles calculations, we discover that gray tin becomes a Dirac semimetal under tensile uniaxial strain, in contrast to a topological insulator under compressive uniaxial strain as known before. In this newly found Dirac semimetal state, two Dirac points which are tunable by tensile [001] strains, lie in the $k_z$ axis and Fermi arcs appear in the (100) surface. Duo the low carrier concentration and high mobility of gray tin, a large chiral anomaly induced negative magnetoresistance and a strong SdH oscillation are anticipated in this half of strain spectrum. Comparing to other Dirac semimetals, the proposed Dirac semimetal state in the nontoxic elemental gray tin can be more easily manipulated and accurately controlled. We envision that gray tin provides a perfect platform for strain engineering of chiral magnetic effects by sweeping through the strain spectrum from positive to negative and vice versa.

Microwave radiation absorption and Shubnikov-de Haas oscillations in semimetal InAs/GaSb/AlSb composite quantum wells

Strong Shubnikov-de Haas (SdH) oscillations were observed in the derivative of microwave absorption (f=10 GHz) in InAs/GaSb/AlSb composite quantum wells (CQWs) using electron-paramagnetic resonance spectroscopy at low temperatures (2.7 to 20 K) and in the magnetic field up to 14 kOe. CQWs were grown on n-GaSb:Te(100) and n-InAs:Mn(100) substrates with various widths of QWs by MOVPE. A predominant contribution to the bulk n-GaSb substrate in SdH oscillations was manifested. Two frequencies of the SdH oscillations connected with warping of the Fermi surface of GaSb were found from Fourier analysis. An unusual angular indicatrix was observed in dependence on the orientation of the samples grown on n-GaSb in the magnetic field. The obtained results can be explained by bulk inversion asymmetry, which is a feature of substances with a lack of inversion centers. For CQWs grown on n-InAs:Mn substrate, only several SdH oscillations with higher period were observed. We succeeded in extracting a contribution of the two-dimensional carriers of InAs QW∼H⊥, where H⊥=H·cos Θ, from bulk substrate oscillations using a special spline interpolation from the angular dependence of SdH oscillatory amplitudes in the angle range of 0 to 90 deg.

Microwave radiation absorption by 2D-electrons in the type II composite InAs/GaSb/AlSb quantum wells in a magnetic field

Strong Shubnikov - de Haas (SdH) oscillations were observed in the derivative of microwave absorption (f = 10 GHz) in the InAs/GaSb/AlSb composite quantum wells (CQWs) using electron-paramagnetic-resonance spectroscopy at low temperatures (2.7-20 K) and in the magnetic field up to 14 kOe. CQWs were grown on the n-GaSb:Te(100) and n-InAs:Mn(100) substrates with various width of QWs by MOVPE. Predominance contribution of the bulk n- GaSb substrate in SdH oscillations was manifested. Two frequencies of the SdH oscillations were found from Fourier analysis, which is connected to warping of the Fermi surface of GaSb. Unusual angular indicatrix was observed in dependence on orientation of the samples in the magnetic field. Obtained results can be explained by inversion asymmetry, which is a feature of the substances with lack of inversion centres. For CQWs grown on n-InAs:Mn (ns = 1.1 × 1017 cm-3) substrate, only several SdH oscillations with higher period were observed. Taking into account isotropic Fermi surface of bulk InAs, we succeeded to extract a contribution of the 2D carriers of InAs QW ∼ H⊥, where H⊥= HconstcosΘ, from bulk substrate oscillations using special spline interpolation from angular dependence of SdH oscillatory amplitudes in the angle range 0-90°. 2D electron concentration in the InAs QW ns ≈ 1 - 3 × 1011 cm-2 was evaluated from oscillatory period.

Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2.

Layered compounds AMnBi2 (A = Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm2V−1S−1) and a π Berry phase accumulated along cyclotron orbit. Compared with AMnBi2, BaMnSb2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120 K and the ratio of the out-of-plane and in-plane resistivity reaching ~670. Additionally, BaMnSb2 also exhibits a G-type antiferromagnetic order below 283 K. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.

Extremely large magnetoresistance and ultrahigh mobility in the topological Weyl semimetal candidate NbP

Weyl semimetals are predicted to exhibit a host of unusual transport properties. NbP, a system predicted to share characteristics of both normal and Weyl semimetals, is now shown to have a very large, non-saturating magnetoresistance. Recent experiments have revealed spectacular transport properties in semimetals, such as the large, non-saturating magnetoresistance exhibited by WTe2 (ref. 1). Topological semimetals with massless relativistic electrons have also been predicted2 as three-dimensional analogues of graphene3. These systems are known as Weyl semimetals, and are predicted to have a range of exotic transport properties and surface states4,5,6,7, distinct from those of topological insulators8,9. Here we examine the magneto-transport properties of NbP, a material the band structure of which has been predicted to combine the hallmarks of a Weyl semimetal10,11 with those of a normal semimetal. We observe an extremely large magnetoresistance of 850,000% at 1.85 K (250% at room temperature) in a magnetic field of up to 9 T, without any signs of saturation, and an ultrahigh carrier mobility of 5 × 106 cm2 V−1 s−1 that accompanied by strong Shubnikov–de Haas (SdH) oscillations. NbP therefore presents a unique example of a material combining topological and conventional electronic phases, with intriguing physical properties resulting from their interplay.

Observation of oscillatory magnetoresistance periodic in1∕BandBinCa3Ru2O7

We report magnetoresistance oscillations in high magnetic fields, B, up to 45 T and over a wide range of temperature in the Mott-like system Ca3Ru2O7. For B rotating within the ac-plane, slow and strong Shubnikov-de Haas (SdH) oscillations periodic in 1/B are observed for T≤1.5 K in the presence of metamagnetism. These oscillations are highly angular dependent and intimately correlated with the spin-polarization of the ferromagnetic state. For B||[110], oscillations are also observed but periodic in B (rather than 1/B) which persist up to 15 K. While the SdH oscillations are a manifestation of the presence of small Fermi surface (FS) pockets in the Mott-like system, the B-periodic oscillations, an exotic quantum phenomenon, may be a result of anomalous coupling of the magnetic field to the t2g-orbitals that makes the extremal cross-section of the FS field-dependent.

Field-induced metal-insulator transition in a two-dimensional organic superconductor.

The quasi-two-dimensional organic superconductor beta"-(BEDT-TTF)2SF5CH2CF2SO3 (Tc approximately 4.4 K) shows very strong Shubnikov-de Haas (SdH) oscillations which are superimposed on a highly anomalous steady background magnetoresistance, Rb. Comparison with de Haas-van Alphen oscillations allows a reliable estimate of Rb which is crucial for the correct extraction of the SdH signal. At low temperatures and high magnetic fields insulating behavior evolves. The magnetoresistance data violate Kohler's rule, i.e., cannot be described within the framework of semiclassical transport theory, but converge onto a universal curve appropriate for dynamical scaling at a metal-insulator transition.

Growth and characterizations of GaN on SiC substrates with buffer layers

High quality GaN epitaxial layers were grown on 6H–SiC substrates by using low-pressure metalorganic chemical vapor deposition method. Samples employing a three-period GaN/Al0.08Ga0.92N (100 Å/100 Å) as a buffer layer produce a good quality GaN epitaxial layer, with mobility and carrier concentration of 612 cm2/V⋅s and 1.3×1017 cm−3 (at 300 K), respectively. The enhanced electron mobility in the Al0.08Ga0.92N/GaN heterostructures is also observed. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility at 4.2 K for the Al0.08Ga0.92N/GaN heterostructure are 5.8×1012 cm−2 and 5300 cm2/V⋅s, respectively. Strong SdH (Shubnikov–de Haas) oscillations were observed to confirm the two-dimensional electron gas (2DEG) phenomenon at the AlGaN/GaN top heterointerface. In addition, an extra SdH oscillation also resulted from the high-quality 2DEG channel of the GaN/AlGaN bottom heterointerface.

Quantum and semi-classical magnetoresistance oscillations in a new organic metal (BEDT-TTF) 2T1Hg(SeCN) 4

Magnetoresistance studies of a new quasi-two- dimensional organic metal, (BEDT-TTF) 2T1Hg(SeCN) 4, are presented. Angular oscillations of the semi-classical magnetoresistance and very strong Shubnikov-de Haas (SdH) oscillations have been found both associated with an extremely weakly warped Fermi surface cylinder with the cross-sectional area amounting to 16% of the first Brillouin zone. The shape of the Fermi surface cross-section has been reconstructed using the measured periods of the angular magnetoresistance oscillations. The spin-splitting effect manifested in a non-monotonous angular dependence of the SdH oscillations points to the importance of many-body interactions in the system.